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The benefits in using standardised designs for signalling installations have long been recognised. As technology advances, so too have the methods and opportunities available to utilise these efficiencies.

Throughout the history of signalling, various levels and methods of standardised design have been deployed:
• Mechanical interlockings using a standard lever and locking layouts;
• Electro-mechanical systems with geographic modules and typical circuits;
• Computer-Based Interlockings (CBIs) with built-in sub-routines and background functions.

In each instance, both benefits and impediments arose. Within the Processor-Based Interlocking (PBI) era, there has been a drift towards the provision of “rule sets”, which are closed to the signal designer; however, provide consistency and efficiency. Rule sets may be locked to a particular PBI, however, may also take the form of a specification which can be applied on many processor-based platforms. Key considerations, therefore, include requirements definition, validation, traceability, security and documentation.

This paper explores the options, considerations and benefits available in developing and using reusable and background
code in modern signalling systems.